Coated paper



Patented Mar. 17, 1936 UNHTED STATES COATED PAPER Maine No Drawing.Original application October 26,

1932, Serial No. 639,735. Divided and this application April 10, 1933,Serial No. 665,339

6 Claims.

' vention of this application relates to the paper coated with the saidcomposition.

The coated paper which is the subject of this invention combines anumber of desirable characteristics which have not heretofore beenattainable together in one product. The nature of coatings has been suchthat certain characteristics could only be obtained at a sacrifice ofothers.

The coating itself as disclosed and claimed in my said copendingapplication is of course responsible for the finished characteristicspresented by the paper. Among its advantages pointed out in thatapplication is the fact that the coating is of high whiteness and iseasily polished.

For example it is capable ,of being polished with low calender pressuresto a high gloss attainable with previous coatings only with fiint orother special processes. Thus the blackening which results from highcalender pressures may be obviated. Also due to the avoidance of hardcalendering the absorption of the coated paper to printing ink may bekept normal.

Another advantage of the coating is the fact that it is satisfactorilybound for any normal printing process and requires less adhesive thanother comparable coatings, wherefore the coated paper has a higherdegree of flexibility than is afforded by such other coatings.

In addition to the high gloss, the coating when properly treated alsoaffords a surface peculiarly free from roughness or mottle. Thesuperiority of the coated paper in respect to the smoothness of itssurface, in spite of the relatively light calendering required, isreadily apparent on visual examination.

The desirable physical properties of the coated paper to which thisapplication relates include, therefore, excellent brightness of colorwith high opacity, high gloss, absence of blackening, nor- 5 mal inkabsorption, flexibility, freedom from mottle and peculiar smoothness.These and other properties will be further elaborated hereinafter.Themethod for producing the coated sheet, which method constitutes apartof this inven- 50 tion, includes forming a coating composition byprecipitating a compound pigment directly upon a base pigment, coating apaper with the composition so formed, and calendering the dried coatwith a light pressure. Specifically, in the pre- 55 ferred embodiment, awhite coating is produced by precipitating calcium sulphate and aluminumhydroxide directly upon particles of a base pigment, such as claysuspended in water, and adding an appropriate adhesive such as caseinthereto. The coating mixture thus prepared is ap- 6 plied to paper stockand dried thereon. Thereafter the coated paper is calendered with alight pressure.

In carrying out the .process in this particular form, lime, alum andclay will be kept on hand 10 in dry form and when the coating is to beprepared a suspension of the clay in water will be produced, and limeincorporated therewith to form a slip or suspension of clay and calciumhydroxide. The respective materials are well 15 mixed and thoroughlyincorporated togetherv and then alum in solution is added to the slip,followed by a small amount of alkali, causing a precipitation of calciumsulphate and aluminum hydroxide in the presence of the clay particles.It 20 is probable that the particles of clay are thoroughly enveloped inthe enrobing layer of voluminous aluminum hydroxide carrying theIcalciumsulphate precipitate. The material thus .produced makes the potentialnew and useful prop- 25 erties in the coating when the material isapplied upon paper.

In preparing the product of this invention and in practicing theprocess, the following materials in substantially the proportions andmanner given may be used, it being understood that this is merelyillustrative and that the proportions may vary substantially accordingto the precise results desired:

Parts 35 Cold water '75 English china clay 50- Hydrated lim 14 Water 234o Alum 17 Water' V '7' Soda ash 0.5 Ammonia ,0.2

, f Water 55 Casein -i 15 The process may be begun by mixing the coldwater and the clay in appropriate container with the hydrated lime; Thismixture should be stirred for about twenty minutes, thatis,,stirringshould be begun when the water is in the mixing vessel, and continuedfor a somewhat "longer time than is required for the addition of the drysolids to the container. The alum should be thetemperature to about 65C.

. preferably two hours.

The soda ash is to be dissolved in the third quantity of water at aboiling temperature and added to the composition at the end of the twoand a half hours of stirring. Simultaneously with the addition of thesoda ash, cooling water in a coil or jacket of the mixing vessel shouldbe turned on and the preparation cooled as rapidly as possible to 25 C.When this-temperature is reached, the ammonia of 26 Baum or a specificgravity of 0.8974 is added, and the stirring continued for five minuteslonger.

The casein solution is separately made in substantially the quantityindicated and after the ammonia is added to the mixture and the mixturestirred the required five minutes, 14 parts of the casein solution areadded to the preparation, and the stirring continued for at least onehour and At the end of this period the remainder of the casein solutionis added. The preparation is then ready for use. If, however, it is notrequired for immediate use, or the entire quantity is not transferred atonce to the coating machine, stirring should be continued until thepreparation has been entirely'removed from the mixing vessel. It appearsthat the continuance of the stirring is desirable to maintain thehomogeneity of the preparation, and to maintain other of the desirableproperties;

The material prepared by the process above described is a'verysatisfactory coating material having desirable physical propertiessurpassing those of other coating preparations. The fact that theprecipitation of the white pigment material has occurred in the presenceof the clay results in a more thorough distribution of the pigment, eachparticle of the filler being more completely coated with the pigmentthan is possible to efiect by stirring previously formed pigment into aclay suspension in water.

It may be noted that a small proportion of soda ash is added to themixture after the primary reaction is complete. It is found that theaddition of this small quantity of soda ash produces beneficial resultsby improving the obtainable polish, increasing the smoothness of thefinished paper sheet, and improving the ink absorption. The ammoniaadded with the soda ash is found to improve the working qualities of theprepared pigment on the coating machine, and to minimize settling of thepigment. It also appears to have a beneficial effect upon the adhesive,and to improve the physical state of the pigment precipitate.

The coating mixture as thus prepared is then applied to paper stock inthe usual way in the usual type of coating machine, and dried in theusualway.

After drying the material may be calendered in the usual type'ofcalendering roll stand, but

the response of the coating to calendering operations is quite differentin character from that of other coatings. The obtained gloss is higherthan that obtainable by simple super-calendering of other coatingpreparations suitable for coating job printing paper. This extremelyhigh gloss combined with excellent smoothness is also obtained by acalender roll pressure which is substantially less than that used forcalendering other white coatings, and is accompanied by no blackening ordiscoloration. Furthermore, the complete coating in spite of theextremely high gloss is well bound, and has rapid absorption of printingink, a property not usually characteristic of highly calendered glosscoatings.

It has been found possible to obtain a glass of 90% and even higher asmeasured on the Ingersoll glarimeter, without blackening. The resultingsurface is substantially free from roughness or mottling' of any kind.

The main advantage of the coating in producing the characteristicproperties of the coated paper is considered to result from theprecipitation of a pigment of high whiteness directly upon a bulky basepigment, and so far clay as a base pigment and alum and lime as thereacting agents ive the best results. It is, however, to be understoodthat other reacting agents may be used. Other base pigments than claymay also be used, such as barytes, zinc oxide, lithopone, etc. Likewise,other alums or sulphates may be used,such as magnesium sulphate and zincsulphate. If a colored coating is desired, it is of course possible tosubstitute other alums or sulphates of metals forming colored compoundssuch as iron, chromium, copper, etc. Likewise, the lime as calcium oxidemay be replaced by dolmitic lime containing magnesium, as well as byother'substances which wfll be obvious to those skilled in the art.Certain other desirable effects may also be obtained by the addition ofsmall quantities of certain other pigments. For instance, smallproportions of titanium dioxide, or preparations containing titaniumdioxide, may be added to the mixture at an appropriate stage in thepreparation thereof. Likewise talc and other equivalent substances alsoproduce desirable eflects for cer- I of an alum, adding an alkali to thereacting mixture, adding an adhesive to the mixture, and coating thecomposition upon paper.

2. The process of preparing a coated paper comprising the steps ofmixing clay in water, adding thereto lime to produce a shiny, thereafteradding an lum in solution and an alkali to precipitate acalcium-aluminum compound pigment upon the clay, further adding anadhesive solution thereto and coating the composition upon paper.

3. The process of preparing a coated paper comprising the steps ofmixing clay in water, adding thereto lime to produce a slurry,thereafter adding an alum in solution and an alkali topmcipitate acalcium-aluminum compound pigment upon the clay, further adding anadhesive solution thereto, coating the composition upon paper, andcalendering the dried coat with a light pressure.

4. A paper made by the process of mixing to-' ing an alkali to thereacting mixture, adding an adhesive to the mixture, and coating thecomposition upon paper.

5. A paper madeby the process of mixing clay in water, adding theretolime to produce a slurry, thereafter adding an alum in solution and analkali to precipitate a calcium-aluminum compound pigment upon the clay,further adding an adhesive solution thereto and coating the compositionupon paper.

6. A paper made by the process of mixing clay in water, adding theretolime to produce a slurry, thereafter adding an alum in solution and analkali to precipitate a calcium-aluminum compound pigment upon the clay,furtheraclding an adhesive solution thereto, coating the compositionupon paper, and calendering the dried coat with a light pressure.

CLARK C. HERITAGE.

